1. Field of the Invention
This invention relates to hydraulic circuit for construction machines such as, for example, a hydraulic excavator, hydraulic crane, etc., and, more particularly, to a control system for the hydraulic circuit apparatus adapted to effect a control of actuator speeds by controlling the displacement volume of a hydraulic pump.
2. Description of the Prior Art
One type of hydraulic circuit apparatus of a construction machine such as a hydraulic excavator, hydraulic crane, etc., known in the art comprises at least first and second variable displacement hydraulic pumps, at least first and second hydraulic actuators driven by the first and second hydraulic pumps, and valve means for controlling hydraulic connections between the hydraulic pumps and the actuators. In this type of hydraulic circuit apparatus, the speeds of the first and second actuators are controlled by controlling the displacement volumes of the first and second hydraulic pumps, with the driving directions of the first and second actuators being preferably controlled by controlling the delivery directions of the first and second hydraulic pumps, and with the first actuator being driven by both the first and second hydraulic pumps by controlling the valve means. However, in the control system of the prior art the problem arises that, when the first actuator is driven by both the first and second hydraulic pumps, acceleration or deceleration of the first actuator undergoes stepwise abrupt changes after operation of the first actuator is initiated until its speed becomes constant and after reduction in speed thereof is initiated until it is brought to a halt, so that the circuit apparatus exhibits poor operational characteristics and a great force of shock is exerted on the machine. In a control system proposed in an effort to avoid this stepwise abrupt change in acceleration or deceleration, it is imperative that when it is desired to drive the second actuator by means of the second hydraulic pump by actuating the valve means when the first actuator is driven by both the first and second hydraulic pumps, actuation of the valve means be effected after rendering the displacement volume of the second hydraulic pump zero in order to avoid a shock that might otherwise be given to the actuators. Thus, the second actuator might not become operative immediately at the time the operation lever is manipulated and there might be a time lag in starting operation of the second actuator. Also, the hydraulic pumps might have a high incidence of changes in displacement volumes.